In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, introduction of a dual connectivity method (Dual connectivity) in Release 12 and thereafter is expected (see Non Patent Document 1). In the dual connectivity method, a user terminal simultaneously establishes a connection with a plurality of base stations. Because a radio resource is assigned to the user terminal from each base station, it is possible to expect an improvement in throughput.
In the dual connectivity method, of the plurality of base stations that establish a connection with the user terminal, only one base station (hereinafter, called “master base station”) establishes an RRC connection with the user terminal. On the other hand, of the plurality of base stations, another base station (hereinafter, called “secondary base station”) provides an additional radio resource to the user terminal without establishing an RRC connection with the user terminal. It is noted that the dual connectivity method may also be called an inter-base station carrier aggregation (inter-eNB CA).